1. Field of the Invention
The present invention generally relates to a magnetic shield device comprising at least one cylinder whose both ends are opened, a space within said at least one cylinder being shielded from ambient magnetic fields.
The present invention relates more particularly to a magnetic shield device comprising an open-ended cylinder made of a non-magnetic material, a ribbon which is made of a magnetic material having a rectangular hysteresis loop and is wound and fixed around the cylinder, and a coil wound around the cylinder, wherein by conducting a magnetic shaking current to the coil, an inner space of the cylinder is free from external magnetic fields.
Furthermore, the invention relates to a magnetic shield device comprising one or more open-ended cylinders arranged concentrically, said cylinder or cylinders being suitably configured to construct, therein, a space which is free from external magnetic fields.
2. Description of the Related Art
In many applications, it is required to measure a small change of a magnetic field without being affected by external magnetic fields. In usual magnetic field measurement, biological magnetic field measurement, particularly brain magnetic field measurement, and precise physical measurement, a space free from environmental magnetic noise such as geomagnetic noise and environment magnetic noise is required for measuring a weak magnetic field measurements as well as for evaluating a performance of a precise flux meter.
Heretofore, a magnetic shield device having a space completely covered with a magnetic material has been widely used. However, since the space is completely covered, working performance is low, and there is a problem that an arrangement of a substance within the space could not be known accurately.
In order to solve such problems, there has been proposed an open-ended cylindrical magnetic shield device, in which a ribbon made of a magnetic material having a rectangular hysteresis loop is fixed on an outer surface of an open-ended cylinder made of a non-magnetic material or PERIMALLOY, which is a trademark used for any of several alloys of nickel and iron having high magnetic permeability. Such an open-ended cylindrical magnetic shield has a simple structure, and has been used to measure a weak magnetic field. The magnetic shield has been designed to attain a high shielding factor S, which is represented by a ratio of a ambient magnetic field intensity H.sub.0 to a leaked magnetic field intensity H.sub.1 within the shield (S=H.sub.0 /H.sub.1).
One of the inventors of present invention suggested, in Japanese Patent Application Kokai Hei 3-66839, a magnetic shield device, in which a ribbon made of a magnetic material with a rectangular hysteresis loop is wound and fixed around an outer surface of the open-ended cylinder, i.e. shell, a coil is wound around the cylinder, and a magnetic shaking current in a frequency range from 150 Hz to 10 KHz is supplied to the coil.
In such an open-ended cylindrical shield device, there has been proposed to make the cylinder of a non-magnetic material such as aluminum or copper, and a ribbon made of a magnetic material such as Metglass 2705M (trademark of Allied Corp. in USA) with a rectangular hysteresis loop is wound helically, i.e. in a solenoid-shape around the open-ended cylinder. A magnetic shaking enhancement is generated by supplying a current to the coil wound toroidally around the cylinder.
In this magnetic shield device, it is possible to attain the shielding factor S more than 100 by using a small amount of magnetic material, but recently a higher shielding factor S is required. However such a high shielding factor S could not be obtained by using this structure. According to the publication mentioned. It has been proposed in the above mentioned Patent Publication to increase the number of winding layers of magnetic ribbon, and to arrange a plurality of cylinders concentrically.
In addition, in the magnetic shield device mentioned in the above Patent Publication, the ribbon made of a magnetic material with a rectangular hysteresis loop is wound and fixed helically, i.e. in a solenoid-shape around the cylinder. Therefore, there is a problem that this structure could generate a high magnetic shaking enhancements on the lateral, i.e. transverse direction, but extremely low on the axial direction.
On the other hand, the open-ended cylindrical magnetic shield device consisting of a PERMALLOY has been also proposed. Upon designing such a magnetic shield device, it has been mainly aimed to increase the shielding factor S as explained above, but has not been aimed to make a residual magnetic field within the shell to be uniform. Thus in the magnetic shield device employing the open-ended cylinder, not only the shielding factor S but also the uniformity of residual magnetic field are important characteristics.
For instance, in the measurement of an extremely weak magnetic field such as the brain magnetic field, a first order differential type pick-up (called gradiometer) has been widely used, because an effect of the uniform residual magnetic field could be cancelled out. If the residual magnetic field within the shell has a gradient, the pick-up might respond to such a gradient, and the magnetic field could not be precisely measured. Under such a circumstance, it has been desired to develop a magnetic shield device comprising both ends opened cylinder or cylinders, in which a high shielding factor could be attained and a residual magnetic filed within the shield device has an excellent uniformity.